Means for comparing or measuring the constants of electric circuit elements



Dec. 9, 1952 MERGER 2,621,229

MEANS FOR COMPARING OR MEASURING THE CONSTANTS OF ELECTRIC CIRCUIT ELEMENTS Filed Feb. 26, 1948 INVENTOR I MARC MERC/ER AGENTS Patented Dec. 9, 1 952 MEAN S FOR COMPARING OR 'M-EA'SURING THE CONSTANTS OF ELECTRIC CIRCUIT ELEMENTS Marc Mercier, Paris, France Application February 26, 1948, Serial No..11,228 :In EranceJanuary 9, 1942 Section 1, Public Law 690,, August 8,1946 Patent expires January 9, 1962 3 Claims. 1

The present invention relates to circuit ar rangements and means for the measurement of electric magnitudes.

More specifically the invention is directed to the provision of combinations of circuits, e; g. electric discharge circuits, which make it possible to compare or measure the inductance in electric circuits.

The main object of the invention is to provide the element to be measured as a component part of an oscillatory circuit and to render the measurements or comparisons simultaneously rapid, practical and commercial in character, while also to increase the sensitivity and to maintain -accuracy in said measurements.

Another object f the invention is to provide means for effecting a direct reading on the apparatus, without any calculation, of the value of the measured magnitude.

Another object of the invention is to provide that the adjustment of the device may at will be effected by audible or visual means according to convenience.

Another object of the invention is to provide a dial graduation for the scale of the magnitude to be measured, which is adapted to be engraved beforehand.

Another object of the invention is to provide a measuring circuit of the type described, wherein the influence of the distributed capacitance isnegligible.

Another object of the invention is to provide a measuring device of the character described which allows to effect very rapid and accurate measurements on series of elements having appreaching values, with a direct reading of the percentage deviation with reference to an element selected as standard or reference.

A further object of the invention is to provide a measuring device of the character described which allows to compare very rapidly, sensitively andaccurately the damping of the circuits subjected to measurements, and thereby to ascertain the quality thereof or detect any defects therein.

Other objects, features and advantages of the invention will appear from the ensuingdisclosure.

Various forms of embodiment of the invention are illustrated. in the accompanying drawings given merely by way of example and not of limitation and wherein:

Fig. 1 illustrates in block-schematic showing a form of the invention more particularly adapted for use in measuring inductances.

Fig. 2 illustrates a wiring diagram of the device 2 more specifically adapted for measuring inductances and whereof the block-diagram is indicated in Fig. 1.

Fig. 1 schematically illustrates in block-diagram form a device designed for the measurement of an induction coeflicient Lac.

For this purpose, the inductance La: is connected across a capacitor .assembly which com prises a fixed capacitor C, an adjusting capacitor C2 and a Vernier capacitor C l and forms an oscillatory circuit B therewith. :Said fixed capaci'tor C is, according to the invention, selected at a high capacitance value so that with respect thereto the distributedcapacitance of the inductance to be measured and of the connections thereof will be negligible. A small resistance a is arranged in series with the inductance, La: to receive from the oscillator O the necessary energy to generate oscillations in the oscillatory circuit B. Across the oscillatory circuit B there is arranged a detecting and amplifying unit DA the output of which may supply either an audio-- receiver T or a rectifier and an indicating instru ment E.

The operation of the device will then easily be understood:

The inductance Lac will impose on the oscillatory circuit formed by said inductance itself and the capacitance Co=C+cl+c2 a certain wave length value A. When the oscillator 0 is tuned to said wave length, the oscillatory circuit 3 is energized and the audio-receiver T will emit a tone signal, because the oscillator O is modulated by a modulator and the indicating instrument E will show a deviation, which will reach a margimum value for a ).-value such that ?\=21r\/ LrrCo wherein:

Co is a constant, since C is a constant,

(31 in this measurement is maintained at a constant value equal to one half the maximum value and 02 is a constant.

It will thus be seen that we may writewherein K is a constant. It is therefore possible to calibrate the dial M of the oscillator 0 directly in inductance values and the apparatus can thus provide a direct reading, the device for ascertaining the maximum being at will an optical or an acoustical one. Any known device of the threshold type may be used in the amplifier detector unit DA to increase the acuteness of the maximum as much as may be desired. For the measurement of La, the capacitor cl is adjusted to its intermediate or center value.

Should it now be desired to check the uniformity of a iactori production, and specifically to measure the percentage deviation from one inductance element to a standard one or the first one in the production, it will simply be necessary without altering the adjustment of the dial M in the oscillator O, to replace the first inductance Lwo by another one Lwl and adjust the capacitor 02 so as to observe a maximum indication in the instruments T or E.

If the dial of the capacitor cl is calibrated in scale calibrations, each corresponding for instance to on each side of the middle zero position, then:

If the capacitor cl shows an increase of two calibrations, the inductance Lscl is 2% smaller than the inductance Lace, and

If the capacitor cl shows a decrease of two graduations, the inductance Lac! is 2% larger than the inductance Lro in correspondence with the equation: LxCo=constant=K Fig. 2 illustrates by way of example and not of limitation an electric wiring diagram of a device according to the invention for inductance measurements, already shown in block schematic form in Fig. 1 and which will now enable a more detailed description of some features of the invention to be made. The sources of electric power supply which may either comprise batteries or supply devices from the direct or alternating current network will be so designed as to provide a highly stable supply voltage. In Fig. 2 said sources have not been shown. The negative polarity of the high voltage source is assumed to be connected with the earth or ground of the apparatus. The filaments fl, f2, f3, id of the tubes VI, V2, V3, V4 are connected with heating circuits not shown. The oscillator should be very stable in frequency. For this purpose, it may be formed for example of a triode V2 connected with the terminals of an oscillatory circuit comprising for instance a variable capacitor M and inductances such as a, b; the cathode connection is efiected through the common point of the inductances such as a: and b. The grid resistance 1 is short-circuited with respect to high frequency currents by the capacitor 8. A modulator H comprises for instance a triode VI and a large air-gap iron core carrying the inductances 2 and 3 respectively tuned to the desired and audio-frequencies by means of the capacitors l and 6. The capacitor 6 moreover shunts the anode of the tube V2 with earth for the high frequencies produced by the oscillator O. The resistor 4 forms the grid resistor for the oscillator H. By placing this resistor preferably in the cathode circuit, it is possible, by adjusting its value, to control the degree of modulation, and the counter-feed-back thus produced, is effective to eliminate harmonics. The potentiometer system formed by the three resistors 9, I0 and I 1 provides for the anode supply for the oscillators H and O and the variable tap P makes it possible to adjust the magnitude of the modulated oscillations supplied from the oscillator 0. However, the just described potentiometer system can be replaced by a series resistance. The capacitor 12 forms part of the supply filter circuit. The capacitor 5 provides for short-circuiting the potentiometer or resistance for audio-frequencies.

l'he above described circuit arrangement makes it possible to ensure that the oscillator 0 will have a constant degree and frequency of modulation.

The energy supplied from the oscillator O is picked up through a winding such as d which is coupled to an inductance 12. Various sets of inductances such as at, 0!, di; a2, b2, d2, etc. may be substituted for the inductances a, b, d, so as to make available the desired number or frequency ranges. For this purpose, a switch provided with contacts Hi, It, to, H is provided Ior switching from one set for inductances to another, while the movable contact 13 is adapted to earth those inductances which are not in use. All the circuits comprising the oscillator O and the circuits B are enclosed in a shielding structure not shown and thermally insulated II'OIl'l. all or the heatgeneratlng portions or the apparatus, so as to ensure constancy in the measurements. It may be or interest to note:

1. That an additional inductance Z may be incorporated in the apparatus in series with Lzc, and the sum total or the residual inductances of the apparatus together with this additional mductance is, by suitable ad ustment, caused to assume a constant value, the scale gradua-- tion or the dial in being displaced in a suitable direction by the amount or this value, whereby Lac may be measured down to values as low as zero.

2. Through the use of a switch it becomes possible to alter the operating conditions of the cathode circuit so as to vary the character of the high-frequency detector, in order to operate the detector out of saturation conditions, and also to alter the sensitiveness oi the apparatus in the case or inductances hat having widely cufiering surtension coemcients.

3. The audio-detector T may be supplied from an individual tube.

'lhe line 18 having a very low impedance, transmits the high Irequency energy to the oscillatory circuit 15 comprising the inductance La: which it is desired to measure. Said oscillatory circuit B comprises a hxed capacitor C of relatively high value, 93 ,uF by example, so as to cause any distributed capacitance to be negligible, an adjusting condenser C2 and the vernier condenser Cl having a zero intermediate position.

through capacitor 19 and resistor 28 for forming the detector D.

The resistor 24 shunted by the condenser 23 supplies the screen grid, and the resistor 25 forms the anode circuit. The capacitor 26 transmits the detected frequency to the amplifier A comprising the tube V i, the cathode of which is biassed through a resistor 29 shunted by the capacitor 28 and the grid of which is provided with a leak resistor 21. The anode circuit of said tube comprises in series an inductance 3B and the audio-receiver 31, adapted to be shortcircuited through the switch 32. The capacitor 33 transmits the amplified audio-currents to the indicating instrument E comprising a rectifier 3B and a galvanometer 35. The above described device operates as follows:

The inductance L1: to be measured is arranged across the terminals 2|, 22 of the oscillatory circuit B. The dial of the capacitor Cl is set to Across the terminals of said oscillatory circuit B there is connected the pentode V3- zero position, corresponding to the capacitance value The particular range of the switch l4, l5, [6, ll in which the oscillator may be .caused to resonate with the circuit B, is found by rotating the variable capacitor M until a maximum indication is reached .either in the audio-receiver T or in the galvanometer 35. As previously indicated, by virtue of the equation: \='21.\/LrCo, the dial M is provided with a scale graduation directly calibrated in values of La. There will be as many such scales as there are-sets-of inductances such as-a, b, d, that is, as there are ranges in the oscillator 0. The capacitor 02 is an adjusting element enabling adjustment of the value Co=C+cr+ c1+c2, wherein Or is the total residual capacity of the circuit B, to a .constant value, in each individual apparatus. The capacitor cl is calibrated about a central position of its dial marked as zero, in percentages of the total capacity Co.

If now it is desired to compare various inductances such as L$1,L.1:2 and so on, to an inductance L0 in the same range of values, taken as a standard or reference, the operation is as follows:

1. The inductance L0 is placed across the terminals 2l22.

2. The capacitor C1 is set to the zero calibration.

3. The oscillator O is adjusted as previously indicated, thatis for instance it is so adjusted that the deviation-of the galvanometer 35 will be a maximum; then the value of L0 is read on the dial M corresponding to a wave length A0 of the oscillator O.

4. The inductanceLo is replaced by the inductance Lzri and, without actuating the dial M, the dial of the capacitor C1 is rotated until a maximum deviation is obtained in the galvanometer 35.

A reading is then made on the calibrated dial of the capacitor C1. This reading then provides the percentage deviation of the inductance Lari with respect to the inductance L0. This deviation is negative if 01 has been increased and it is positive if 01 was reduced, with reference to the equation:

io=constant=2 LoC'o=21r /LwCr wherein Cx=C+C2+Cr+ClI Cl$ being the new value of 01).

Moreover, the reading of the maximum deviation on the scale of the galvanometcr 35 makes it possible to compare the qualities of the inductances L0, L23, etc., of approaching values, if the power output supplied by the modulated oscillator OH and the sensitivity of the detector D, amplifier A and reading device E have remained constant from one measurement to the next, this assumption being in fact accurate, in practice, if the various measurements are effected at short time intervals between each other and if the supply voltage of the various elements of the device have remained constant.

The last-mentioned characteristic of the device makes it possible to ascertain any variation in the damping characteristics, that is in practice, immediately to detect, inter-alia, if one or more strands in a multiple wire is or are cut, or if there are any winding turns in short-circuit, or any defective soldered connections, or defective 6, insulating supports, or capacitor with defective clamps (by arranging them in series with Laz).

It will of course be understood that other forms of embodiments of the invention may be constructed, and that various modifications may be made in the above described arrangements, for instance by replacing various electron discharge tubes by multi-electrode tubes or altering the circuit arrangements, without exceeding the scope of the invention.

In particular, the oscillator 0 could .be a nonmodulated one and the detector-amplifier "DA could then comprise an amplifier, a detector and a suitable direct-current amplifier.

What 1' claim is:

1. An apparatus for measuring the inductances :of electric circuit .elemnts comprising, in combination, a modulator; a tunable oscillator supplied by said modulator and having an -adjusting device the scale of which is directly calibrated in inductance values; a coupling oscillatory circuit including a circuit element the indu'ctance of which is to be measured and an open oscillatory circuit having a resistor coupled to said oscillator and one end of which is connected to one end of said circuit element, and aset of capacitors having one side connected to the other end of said circuit element and the other side connected to the-other end of said resistor, said capacitors being mounted in parallel in said set and comprising a fixed capacitor having a high capacitance value for rendering negligible with respect to its own capacitance the distributed-capacitance of said circuit element, and .an adjustable capacitor assembly the capacitance of which is previously adjusted to the value or" the difference existing between a predetermined capacitance vaiue and the sum of the total residualcapacity of said open oscillatory circuit and of the capacitance of said fixed capacitor whereby owing to the constancy of said predetermined capacitance value, a direct reading of the inductance may be made without any computing operation on the scale of said adjusting device, for resonance between said oscillator and said oscillatory circuit; means for detecting the modulated oscillations issued from said oscillatory circuit; and means for indicating the intensity of the thus detected audio-current, the thus obtained modulations giving after detection a large, simple and stable amplification.

2. An apparatus for measuring the inductances of electric circuit elements comprising, in combination, a modulator; a tunable oscillator supplied by said modulator and having an adjusting device the scale of which is directly calibrated in inductance values; a coupling oscillatory circuit including a circuit element the inductance of which is to be measured and an open oscillatory circuit having a resistor coupled to said oscillator and one end of which is connected to one end of said circuit element, and a set of capacitors having one side connected to the other end of said circuit element and the other side connected to the other end of said resistor, said capacitors being mounted in parallel in said set and comprising a fixed capacitor having a high capacitance value for rendering negligible with respect to its own capacitance the distributed capacitance of said circuit element, an adjustable vernier capacitor having a scale the Zero value of which corresponds to the position of said vernier capacitor for which its capacitance is equal to one half the maximum value thereof, said scale being so designed as to indicate percentage deviations with respect to said mid-cae pacitance value, and an adjustable capacitor the capacitance value of which is previously adjusted to the value of the difierence existing between a predetermined capacitance value and the sum of the total residual capacity of the open oscillatory circuit and of the capacitances of the fixed capacitor and of the Vernier capacitor adjusted on said Zero value, whereby owing to the constancy of said predetermined capacitance value, a direct reading of the inductance may be made without any computing operation on the scale of said adjusting device, for resonance between said oscillator and said oscillatory circuit; means for detecting the modulated oscillations issued from said oscillatory circuit; and means for indicating the intensity of the thus detected audiocurrent, the thus obtained modulations giving after detection a large, simple and stable amplification, said Vernier capacitor remaining at said Zero value when measuring the inductance of one electric circuit element and being adjusted for determining in percentage the relative inductance values of a plurality of circuit elements to be compared with said circuit element by di-- rect reading on its own scale.

3. An apparatus for measuring the inductances of electric circuit elements comprising, in combination, a modulator; a tunable oscillator supplied by said modulator and having an adjusting device the scale of which is directly calibrated in inductance values; a coupling oscillatory circuit including a circuit element the inductance of which is to be measured and an open oscillatory circuit having a resistor coupled to said oscillator and one end of which is connected to one end of said circuit element, and a set of capacitors having one side connected to the other end of said circuit element and the other side connected to the other end of said resistor, said capacitors being mounted in parallel in said set and comprising a fixed capacitor having a high capacitance value for rendering negligible with respect to its own capacitance the distributed capacitance of said circuit element, and an adjustable capacitor assembly the capacitance of which is previously adjusted to the value of the difference existing between a predetermined capacitance value and the sum of the total residual capacity of said open oscillatory circuit and of the capacitance of said fixed capacitor, whereby owing to the constancy of said predetermined capacitance value, a direct reading of the inductance may be made without any computing operation on the scale of said adjusting device, for resonance between said oscillator and said oscillatory circuit; means for detecting the modulated oscillations issued from said oscillatory circuit; means for indicating the intensity of the thus detected audio-current, the thus obtained modulations giving after detection a large, simple and stable amplification; and an additional adjustable inductance mounted in series with the circuit element, said additional inductance being previously adjusted so that the sum of its own inductance and of the residual inductance of the oscillatory circuit reaches a constant value, the scale of the adjusting device being displaced in a suitable direction by the amount of this constant value, whereby the inductance of the circuit element may be measured down to values as low as zero.

MARC MERCIER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS McGraw-Hill Book 00., 1933, pp. 50, 51, 52, 218, 219, 245, 246.

Radio World, July 1936, pp. -51. 

